Method of cleaning and forming a negatively charged passivation layer over a doped region

ABSTRACT

The present invention generally provides a method of forming a high efficiency solar cell device by preparing a surface and/or forming at least a part of a high quality passivation layer on a silicon containing substrate. Embodiments of the present invention may be especially useful for preparing a surface of a p-type doped region formed on a silicon substrate so that a high quality passivation layer can be formed thereon. In one embodiment, the methods include exposing a surface of the solar cell substrate to a plasma to clean and modify the physical, chemical and/or electrical characteristics of the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/267,343 [Attorney Docket #: APPM 14745L], filed Dec. 7,2009, which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to the fabricationof solar cells and particularly to a device structure and method ofpassivating a surface of a crystalline silicon solar cell.

2. Description of the Related Art

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon (Si),which is in the form of single, polycrystalline or multi-crystallinesubstrates. Because the cost of electricity generated usingsilicon-based solar cells is higher than the cost of electricitygenerated by traditional methods, there has been an effort to reduce thecost of manufacturing solar cells that does not adversely affect theoverall efficiency of the solar cell.

FIG. 1 schematically depicts a cross-sectional view of a standardsilicon solar cell 100 fabricated from a crystalline silicon substrate110. The substrate 110 includes a base region 101, an emitter region102, a p-n junction region 103, a dielectric passivation layer 104,front electrical contact 107 and rear electrical contact 108. The p-njunction region 103 is disposed between base region 101 and emitterregion 102 of the solar cell, and is the region in which electron-holepairs are generated when solar cell 100 is illuminated by incidentphotons. Dielectric passivation layer 104 may act as an anti-reflectivecoating (ARC) layer for solar cell 100 as well as a passivation layerfor the surface 105 of emitter region 102.

The efficiency of solar cell 100 may be enhanced by use of an ARC layer.When light passes from one medium to another, for example from air toglass, or from glass to silicon, some of the light may reflect off ofthe interface between the two media, even when the incident light isnormal to the interface. The fraction of light reflected is a functionof the difference in refractive index between the two media, wherein agreater difference in refractive index results in a higher fraction oflight being reflected from the interface. An ARC layer disposed betweenthe two media and having a refractive index whose value is between therefractive indices of the two media is known to reduce the fraction oflight reflected. Hence, the presence of an ARC layer on alight-receiving surface of solar cell 100, such as dielectricpassivation layer 104 on surface 105, reduces the fraction of incidentradiation reflected off of solar cell 100 and which, therefore, cannotnot be used to generate electrical energy.

When light falls on the solar cell, energy from the incident photonsgenerates electron-hole pairs on both sides of p-n junction region 103.In a typical solar cell, which comprises an n-type emitter region and ap-type base region, electrons diffuse across the p-n junction to a lowerenergy level and holes diffuse in the opposite direction, creating anegative charge on the emitter and a corresponding positive chargebuild-up in the base. In an alternate configuration, which has a p-typeemitter region 102 and n-type base region 101 (FIG. 1), electronsdiffuse across the p-n junction to form a positive charge on the emitterand holes diffuse in the opposite direction to form a negative chargebuild-up in the base. In either case, when an electrical circuit is madebetween the emitter and the base, a current will flow and electricity isproduced by solar cell 100. The efficiency at which solar cell 100converts incident energy into electrical energy is affected by a numberof factors, including the recombination rate of electrons and holes insolar cell 100 and the fraction of incident light that is reflected offof solar cell 100.

Recombination occurs when electrons and holes, which are moving inopposite directions in solar cell 100, combine with each other. Eachtime an electron-hole pair recombines in solar cell 100, charge carriersare eliminated, thereby reducing the efficiency of solar cell 100.Recombination may occur in the bulk silicon of substrate 110 or oneither surface 105, 106 of substrate 110. In the bulk, recombination isa function of the number of defects in the bulk silicon. On the surfaces105, 106 of substrate 110, recombination is a function of how manydangling bonds, i.e., unterminated chemical bonds, are present onsurfaces 105, 106. Dangling bonds are found on surfaces 105, 106 becausethe silicon lattice of substrate 110 ends at these surfaces. Theseunterminated chemical bonds act as defect traps, which are in the energyband gap of silicon, and therefore are sites for recombination ofelectron-hole pairs.

As noted above, one function of the dielectric passivation layer 104 isto minimize the carrier recombination at the surface of the emitterregion(s) 102 or the base region 101 over which the dielectricpassivation layer 104 is formed. It has been found that forming anegative charge in a dielectric passivation layer 104 disposed over ap-type doped region formed in a solar cell device can help repel thecarriers moving through the solar cell, and thus reduce the carrierrecombination and improve the efficiency of the solar cell device. Whileit is relatively easy to form a passivation layer that has a netpositive charge using traditional plasma processing techniques, it isdifficult to form a stable negatively charged passivation layer on thesurface of a silicon substrate.

Thorough passivation of the surface of a solar cell greatly improves theefficiency of the solar cell by reducing surface recombination. In orderto passivate a surface of solar cell 100, such as surface 105, adielectric passivation layer 104 is typically formed thereon, therebyreducing the number of dangling bonds present on surface 105 by 3 or 4orders of magnitude. For solar cell applications, dielectric passivationlayer 104 is generally a silicon nitride (Si_(X)N_(Y) or abbreviatedSiN) layer, and the majority of dangling bonds are terminated withsilicon (Si), nitrogen (N), or hydrogen (H) atoms. But because siliconnitride (SiN) is an amorphous material, a perfect match-up between thesilicon lattice of emitter region 102 and the amorphous structure ofdielectric passivation layer 104 cannot occur. Hence, the numberdangling bonds remaining on surface 105 after the formation ofdielectric passivation layer 104 is still enough to significantly reducethe efficiency of solar cell 100, requiring additional passivation ofsurface 105, such as hydrogen passivation. In the case ofmulti-crystalline silicon solar cells, hydrogen also helps to passivatethe defect centers on the grain boundary.

During normal processing of the solar cell device the p-type boron dopedregions found in the solar cell may form an oxide layer, such as boronsilicate glass (BSG) layer that is hard to remove prior to forming thedielectric passivation layer 104. The BSG oxide layer may be formed overthe back side of a p-type substrate base region 101, or, alternately,the BSG layer may be formed over a p-type emitter structure. However, itis generally important to remove the formed oxide layer and clean thesubstrate surface to prevent contamination of the solar cell substrateduring subsequent processing and improve the passivating effect of thedielectric passivation layer that is later formed over the substratesurface.

It is also desirable to assure that the solar cell efficiently convertsas much of the optical energy received by the sun into electrical energyas possible. However, since sunlight may be scattered, refracted,diffracted, or reflected fairly easily, several different techniqueshave been developed to enhance light trapping in the solar cells toimprove conversion efficiency. For example, a surface texture may beprovided to increase the surface roughness, thereby assisting the lightto be trapped and confined in the solar cell. Conventional surfacetexturing processes often utilize aqueous alcohol related compounds as achemical source for substrate surface treatment. However, alcoholrelated compounds are flammable, which are fire hazard and be inenvironmental safety concern, thereby requiring special safety measuresduring processing. Also, alcohols evaporate at the temperatures neededto assure that the chemical activity of the etchants in the texturingsolution is in an optimum range to effectively perform the texturingprocess. Evaporation of the alcohol components from the texturing baththus leads to an unstable texturing bath composition when the processesare run at these elevated temperatures.

Therefore, there is a need for an improved method of cleaning asubstrate prior to depositing a passivation layer, an improved method offorming a desirable charge at the surface of the solar cell device tominimize surface recombination of the charge carriers, and there is aneed for a method to form a desirable surface texture on a surface of asolar cell to improve the formed cell's ability to trap incident light.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally provide a method ofpassivating a surface a solar cell substrate, comprising exposing asurface of a p-type doped region formed on a substrate to a first RFplasma that comprises a first processing gas and a first fluorinecontaining gas, and exposing the surface of the p-type doped region to asecond RF plasma that comprises a halogen gas, and RF biasing thesubstrate to form a negatively charged layer on the surface of thep-type doped region.

Embodiments of the present invention may further provide a method ofpassivating a surface a solar cell substrate, comprising exposing asurface of a p-type doped region formed on an n-type substrate to afirst RF plasma that comprises a first processing gas and a firstfluorine containing gas, exposing the surface of the p-type doped regionto a second RF plasma that comprises a second processing gas, a secondfluorine containing gas and hydrogen containing gas, exposing thesurface of the p-type doped region to a third RF plasma that comprises ahalogen gas, and RF biasing the substrate to form a negatively chargedlayer on the surface of the p-type doped region, and depositing a firstsilicon nitride-containing layer on the formed negatively charged layer.

Embodiments of the present invention may further provide a method ofpassivating a surface a solar cell substrate, comprising removing anoxide layer from a surface of a p-type doped region formed on asubstrate, removing a portion of the surface of the substrate that has aconcentration of p-type atoms that is greater than the averageconcentration of the p-type atoms in the p-type doped region, andforming a negatively charged layer on the surface by exposing thesurface to an RF plasma comprising fluorine or chlorine.

Embodiments of the present invention may further provide a method ofcleaning a processing surface a solar cell substrate, comprisingremoving an oxide layer from a processing surface of a p-type dopedregion formed on a substrate, removing a dead region from the processingsurface of the substrate, and forming a negatively charged layer on theprocessing surface.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 schematically depicts a cross-sectional view of a standardsilicon solar cell fabricated from a single or multi-crystalline siliconwafer.

FIGS. 2A-2D depicts cross-sectional views of a portion of a substratecorresponding to various stages of the process illustrated in FIG. 3according to one or more embodiments of the invention.

FIG. 3 depicts a process flow diagram of a passivation layer formationprocess performed on a silicon substrate in accordance with oneembodiment of the invention.

FIG. 4 is a schematic side view of a parallel plate PECVD system thatmay be used to perform embodiments of the invention.

FIG. 5 is a top schematic view of one embodiment of a process systemhaving a plurality of process chambers according to one embodiments ofthe invention.

FIG. 6 depicts a process flow diagram of a passivation layer formationprocess performed on a silicon substrate in accordance with oneembodiment of the invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

The present invention generally provides a method of preparing a surfaceof a silicon substrate and forming at least a portion of a high qualitypassivation layer which is part of a formed high efficiency solar celldevice. Embodiments of the present invention may be especially usefulfor preparing a surface of a p-type doped region formed in a siliconsubstrate, so that a high quality passivation layer structure can beformed thereon. In one embodiment, the methods include exposing asurface of the solar cell substrate to a plasma to clean and modify thephysical, chemical and/or electrical characteristics of the surface.Solar cell substrates that may benefit from the invention includesubstrates that have an active region that contains single crystalsilicon, multi-crystalline silicon, and polycrystalline silicon, but mayalso be useful for substrates comprising germanium (Ge), galliumarsenide (GaAs), cadmium telluride (CdTe), cadmium sulfide (CdS), copperindium gallium selenide (CIGS), copper indium selenide (CuInSe₂),gallilium indium phosphide (GaInP₂), organic materials, as well asheterojunction cells, such as GaInP/GaAs/Ge or ZnSe/GaAs/Ge substrates,that are used to convert sunlight to electrical power.

Embodiments of the invention may also generally provide a method ofcleaning and forming a negative charge containing layer on one or moresurfaces of a substrate, such as a surface of a doped p-type region. Anegative charge containing layer that is formed over a p-type region isgenerally used to repel the electrons flowing in the solar cell deviceand passivate the surface to minimize carrier recombination and improvethe efficiency of the formed solar cell device.

FIGS. 2A-2D illustrate schematic cross-sectional views of a solar cellsubstrate 210 during different stages in a processing sequence used toform a charged layer 219 and a passivation layer 220 on a surface (e.g.,front surface 205) of a solar cell 200. In one embodiment, the chargedlayer 219 and a passivation layer 220 are used together to form highquality passivation layer structure on the solar cell's surface. FIG. 3illustrates a process sequence 300 used to form the passivation layer ona solar cell substrate 210. The sequences found in FIG. 3 correspond tothe stages depicted in FIGS. 2A-2D, which are discussed herein.

FIG. 2A schematically illustrates a cross-sectional view of an at leastpartially formed silicon solar cell 200 that comprises a substrate 210.The substrate 210 includes a base region 201, an emitter region 202, anda p-n junction region 203. The p-n junction region 203 is disposedbetween base region 201 and emitter region 202 of the solar cell, and isthe region in which electron-hole pairs are generated when solar cell200 is illuminated by incident photons of light.

As noted above, during normal processing of a solar cell device a thinand generally poor quality native oxide layer 215 will form on one ormore of the surfaces of the substrate 210. In configurations where thenative oxide layer 215 is formed over a p-type boron doped region, theoxide layer 215 may comprise a boron silicate glass (BSG) containinglayer. In one example, the BSG containing oxide layer is formed over ap-type emitter region(s) 202 formed over an n-type base region 201 (FIG.2A). In an alternate example, the BSG type oxide layer is formed overthe back side of a p-type substrate base region 201. The thickness anddensity of the formed oxide layer 215 will depend on how the layer wasformed, since in some cases the oxide layer may be formed during orafter one or more thermal processing steps, such as a furnace annealstep that used to “drive-in” or anneal one or more layers formed on thesubstrate 210. In some cases the oxide layer may be formed by extendedexposure to air.

Further, it is common for a partially formed solar cell device to alsohave a dead region 216, which contains a high concentration of dopantatoms, formed at the interface between the oxide layer 215 and thesurface of the substrate 210. The high dopant concentration in the deadregion 216 is believed to be created by the diffusion of dopant atoms tothe surface 205 of the substrate 210 during prior doping or thermalprocessing steps. In one example, the dead region 216 contains a highconcentration of boron atoms (e.g., >0.1%) at the surface of a siliconcontaining p-type doped emitter region 202. In one example, the deadregion 216 formed in a silicon substrate contains less than 85%electrically active boron. A p-type dead region contains a higherconcentration of p-type atoms than the average concentration of p-typeatoms in the p-type portion of the substrate below, such as the p-typedoped emitter region 202. In one embodiment, the dead region 216 has adoping concentration high enough to form a region that has a resistivityof less than about 50 Ohm-cm. In general, it is hard to remove theseboron doped layers using conventional processing techniques, which mayinclude wet chemical etching processes. However, it is generallyimportant to perform the cleaning to remove the boron rich layers, whichcontain a high concentration of defects and contaminants, which decreasethe degree of passivation of the interface. These defects may includedislocations, grain boundaries, dangling bonds, and voids; and thecontaminants may include oxygen, silicon and metallic oxides, andmetallic impurities from the bulk Si or from the processing itself.

Referring to FIG. 2A, in one embodiment of the solar cell 200, the baseregion 201 comprises an n-type crystalline silicon substrate, and theemitter region 202 comprises a p-type layer formed over the base region201. While the discussion below primarily discusses a method andapparatus for processing a substrate having a p-type emitter regionformed over an n-type base region this configuration is not intended tolimit the scope of the invention described herein.

At box 302, the surfaces of the substrate 210 is cleaned to remove theoxide layer 215 (FIG. 2A) and dead region 216 (FIG. 2A), and form acharged layer 219 (FIG. 2B) on the surfaces of the substrate. Theprocesses performed at box 302 may be performed in a single processingstep performed in one substrate processing chamber, or as multipleseparate process steps performed in one or more substrate processingchambers. In one embodiment, as shown in FIG. 3, the cleaning processperformed at box 302 includes: 1.) a first cleaning process performed atbox 303 that is used to remove the oxide layer 215, 2.) a secondcleaning process performed at box 304, that is used to remove the deadregion 216, and 3.) a charged layer 219 formation process performed atbox 305. In one embodiment, a substrate 210 may be processed in acluster tool, such as system 500 (FIG. 5), in which one or more of theprocessing chambers are used to remove the oxide layer 215, remove thedead region 216, and form a charged layer 219.

In one embodiment, the first clean process performed at box 303 may beperformed using a batch wet cleaning process in which the substrate 210is exposed to a cleaning solution. In this case, the substrates arecleaned using a wet cleaning process in which they are sprayed, flooded,or immersed in a cleaning solution. The clean solution may be an SC1cleaning solution, an SC2 cleaning solution, HF-last cleaning solution,ozonated water solution, hydrofluoric acid (HF) and hydrogen peroxide(H₂O₂) solution, or other suitable and cost effective cleaning solution.The cleaning process may be performed on the substrate in a roomtemperature bath for between about 5 seconds and about 600 seconds, suchas about 30 seconds to about 240 second, for example about 120 seconds.

In an alternate embodiment, the clean process performed at box 303 maybe performed using a dry cleaning process in which the substrate 210 isexposed to a reactive plasma etching process to remove the oxide layer215. An example of a dry cleaning process and dry processing chamber arefurther described below in conjunction with FIGS. 4 and 5.

Next, at box 304, the dead region 216 is removed from the surfaces ofthe substrate 210 by use of a dry cleaning process. In one embodiment,after removing oxide layer 215 from a surface of the substrate 210 thesubstrate is then delivered to a dry processing chamber to remove thedead region 216. In general, the dry cleaning process performed at box304 generally includes exposing the dead region 216 to an RF ormicrowave plasma for a desired period of time to remove a portion of thesubstrate surface. In one example of the processes performed at box 304,the surface of a silicon substrate (e.g., crystalline silicon substrate)is etched until the surface of the etched region contains at least 85%electrically active boron, and thus the p-type dead region 216 issubstantially removed. An example of such a dry processing chamber anddry cleaning process is further described below in conjunction withFIGS. 4 and 5. One will note that in some cases, it may desirable toassure that the substrate is not exposed to oxygen for extended periodsof time after performing the cleaning process performed at box 303before the cleaning process performed at box 304 is performed on thesubstrate to prevent the re-oxidation of the cleaned surface. Therefore,in some embodiments of the invention, it is desirable to perform all ofthe processes performed at boxes 302-308 in an oxygen-free inert and/orvacuum environment, such as in the vacuum processing regions of acluster tool, or system 500 (FIG. 5), so that the substrate is notexposed to oxygen between the process steps.

At box 305, as shown in FIGS. 2B and 3, the surface 205 of the substrate210 is exposed to a reactive gas containing RF plasma that is used toform a negatively charged layer 219 thereon. In one embodiment, thenegatively charged layer 219 comprises a fluorine (F) rich, and/orchlorine (Cl) rich, layer that is less than about 50 Angstroms (Å)thick. It is believed that by exposing the cleaned surface, such assurface 205, directly to a plasma containing ionized fluorine, and/orionized chlorine, the exposed surface can be “doped”, “stuffed” orcovered with a fluorine rich, or chlorine rich, layer that has anegative charge. In one embodiment, it is desirable for a charged layer219 that is formed on a silicon containing surface to have a density ofnegative charge greater than about 1×10¹² Coulombs/cm². An example of acharge layer formation process is further described below in conjunctionwith FIGS. 4 and 5. While the deposited charged layer 219 will generallycontain a desirable charge density, the charged layer 219 also needs tobe physically, chemically and electrically stable enough to allow one ormore passivation or anti-reflection layer coatings (ARC) to be formedthereon using a PECVD process without significantly degrading theproperties of the formed charged layer 219.

In one embodiment of the process performed at box 305, the surface 205of the substrate 210 is sputtered etched during one or more parts of thecharged layer 219 formation process to help add, or form, surfacetexture on the surface 205. In general, this process may includegenerating a plasma containing an inert gas to sputter a surface of thesubstrate disposed on a biased or grounded substrate support.

In one embodiment, the cleaning process performed at box 304 and thecharged layer 219 formation process, performed at box 305, are completedas one continuous process, or the process performed at box 306.Therefore, in one embodiment, the plasma chemistry used in at least oneportion of the cleaning process performed at box 304 contains afluorine, or chlorine, containing gas, which is used to form the chargedlayer 219 on the surface of the substrate.

In another embodiment, the cleaning processes performed at boxes 303 and304 and the charged layer 219 formation process performed at box 305 arecompleted as one continuous process, such as the process performed atbox 302. Therefore, in one embodiment, the plasma chemistry used in atleast one portion of the cleaning process performed at box 303 and/or atleast one portion of the cleaning process performed at box 304, containsa fluorine, or chlorine, containing gas, which is used to form thecharged layer 219 on the surface of the substrate.

Next, at box 308, as shown in FIGS. 2C-2D and 3, a passivation layer 220is formed on the charged layer 219 using a plasma enhanced chemicalvapor deposition (PECVD) process. In one embodiment, the passivationlayer 220 comprises a plurality of passivation layers, such aspassivation layers 221 and 222, which are used to passivate the surfaceof the substrate. In one embodiment, the passivation layer 220 comprisesa thin passivation and/or antireflection layer that comprises siliconoxide, silicon nitride, amorphous silicon, amorphous silicon carbideand/or aluminum oxide (Al₂O₃). In one embodiment, a silicon nitride(SiN) passivation and antireflection layer, or thin amorphous silicon(a-Si:H) layer or amorphous silicon carbide (a-SiC:H) layer and siliconnitride (SiN) stack is formed over the surface 205 using a chemicalvapor deposition (PECVD) technique on multiple solar cell substratessupported on a suitable large area substrate carrier. In one embodiment,the passivation layer 220 may comprise an intrinsic amorphous silicon(i-a-Si:H) layer and/or p-type amorphous silicon (p-type a-Si:H) layerstack followed by a transparent conductive oxide (TCO) layer and/or anARC layer (e.g., silicon nitride), which can be deposited by use of aphysical vapor deposition process (PVD) or chemical vapor depositionprocess (e.g., PECVD). The formed stack is generally configured togenerate a front surface field effect to reduce surface recombinationand promote lateral transport of electron carriers to nearby dopedcontacts formed on the substrate. An example of a passivation layerformation process is further described below.

Hardware Configuration

FIG. 4 is a schematic cross-section view of one embodiment of a plasmaenhanced chemical vapor deposition (PECVD) chamber 400 in which one ormore of the processes illustrated and discussed in conjunction with FIG.3 may be performed. A similarly configured plasma enhanced chemicalvapor deposition chamber is available from Applied Materials, Inc.,located in Santa Clara, Calif. It is contemplated that other depositionchambers, including those from other manufacturers, may be utilized topractice the present invention.

It is believed that the plasma processing configuration provided in theprocessing chamber 400 has significant advantages over other prior artconfigurations when used to perform one or more of the processesdescribed in FIG. 3. As noted above, in one embodiment, the plasmaenhanced chemical vapor deposition (PECVD) process chamber 400 isadapted to simultaneously process a plurality of substrates. In oneconfiguration, a batch of solar cell substrates is simultaneouslytransferred in a controlled environment, such as a vacuum or an inertenvironment (e.g., transfer chamber 520) to prevent substratecontamination and improve substrate throughput over other prior artconfigurations.

Also, as illustrated in FIG. 5, in the various embodiments of thepresent invention, each batch of substrates 210 are arranged in a planararray for processing as opposed to processing vertical stacks ofsubstrates (e.g., batches of substrates stacked in cassettes).Processing the batches of substrates arranged in a planar array allowseach of the substrates in the batch to be directly and uniformly exposedto the generated plasma, radiant heat, and/or processing gases.Therefore, each substrate in the planar array is similarly processed inthe processing region of a processing chamber, and thus does not rely ondiffusion type processes and/or the serial transfer of energy to allsubstrates in a conventionally configured batch that is being processed,such as a stacked or back-to-back configured batch of substratescommonly found in the prior art.

In one embodiment, the PECVD chamber 400 is configured to process aplurality of substrates at one time. In one configuration, the PECVDchamber 400 is adapted to accept a substrate carrier 425 (FIGS. 4 and 5)that is configured to hold a batch of substrates during the transferringand substrate processing steps. In one embodiment, the substrate carrier425 has a surface area of about 10,000 cm² or more, that is configuredto support a planar array of substrates disposed thereon duringprocessing. In one embodiment, the substrate carrier 425 has a pluralityof recesses (not shown) formed therein that are adapted to hold betweenabout 4 and about 49 solar cell substrates that are 156 mm×156 mm×0.3 mmin size in a face-up or face-down configuration. The substrate carrier425 may be formed from a ceramic (e.g., silicon carbide, alumina),graphite, metal or other suitable material.

The chamber 400 generally includes walls 402, a bottom 404, and ashowerhead 410, and substrate support 430 which define a process region406. The processing region 406 is accessed through a valve 408 such thatthe substrates disposed on the substrate carrier 425, may be transferredin and out of the chamber 400. The substrate support 430 includes asubstrate receiving surface 432 for supporting a substrate and stem 434coupled to a lift system 436 to raise and lower the substrate support430. A shadow from 433 may be optionally placed over periphery of thesubstrate carrier 425. Lift pins 438 are moveably disposed through thesubstrate support 430 to move the substrate carrier 425 to and from thesubstrate receiving surface 432. The substrate support 430 may alsoinclude an embedded heating and/or cooling elements 439 to maintain thesubstrate support 430 at a desired temperature. The substrate support430 may also include grounding straps 431 to provide RF grounding at theperiphery of the substrate support 430. Examples of grounding straps aredisclosed in U.S. Pat. No. 6,024,044 issued on Feb. 15, 2000 to Law etal. and U.S. patent application Ser. No. 11/613,934 filed on Dec. 20,2006 to Park et al., which are both incorporated by reference in theirentirety to the extent not inconsistent with the present disclosure. Inone embodiment, the substrate support 430 has an RF source (not shown)that is coupled to an electrode (e.g., reference numeral 439) that isembedded in the substrate support 430 so that an RF bias can be appliedto the substrates 210 disposed over the substrate support 430.

The showerhead 410 is coupled to a backing plate 412 at its periphery bya suspension 414. The showerhead 410 may also be coupled to the backingplate by one or more center supports 416 to help prevent sag and/orcontrol the straightness/curvature of the showerhead 410. In oneconfiguration, the substrate support 430 and showerhead 410 aregenerally parallel to each other, and in some cases the surface of theshowerhead 410 that contacts the processing region 406 may be slightlycurved, such as concave or convex. A gas source 420 is coupled to thebacking plate 412 to provide gas through the backing plate 412 andthrough the holes 411 formed in the showerhead 410 to the substratereceiving surface 432. A vacuum pump 409 is coupled to the chamber 400to control the process region 406 at a desired pressure. An RF powersource 422 is coupled to the backing plate 412 and/or to the showerhead410 to provide a RF power to the showerhead 410 so that an electricfield is created between the showerhead and the substrate support sothat a capacitively coupled plasma may be generated using the gasesdisposed between the showerhead 410 and the substrate support 430.Various RF frequencies may be used, such as a frequency between about0.3 MHz and about 200 MHz. In one embodiment the RF power source isprovided at a frequency of 13.56 MHz. Examples of showerheads aredisclosed in U.S. Pat. No. 6,477,980 issued on Nov. 12, 2002 to White etal., U.S. Publication 20050251990 published on Nov. 17, 2006 to Choi etal., and U.S. Publication 2006/0060138 published on Mar. 23, 2006 toKeller et al, which are all incorporated by reference in their entiretyto the extent not inconsistent with the present disclosure. It isbelieved that the direct contact of the capacitively coupled plasma tothe processing surface 210A (FIG. 4) of the substrates 210 hasadvantages over designs that do not directly expose all of thesubstrates to the plasma, due to the ability of the chamber 400configuration to provide energetic and/or ionized species directly toall parts of the processing surface 210A during processing. The degreeof plasma interaction applied to the complete processing surface 210Acan be directly controlled in the chamber 400 by adjusting the RF powerdelivered to the showerhead 410, the chamber pressure during processing,and/or the bias applied to the substrate support 430. Typical non-directexposure type chamber configurations include remotely driven plasmaconfigurations or other stacked wafer configurations that rely on thediffusion of the generated plasma to one or more of the substrates, orparts of each of the substrates, during processing.

However, in some embodiments, a remote plasma source 424, such as aninductively coupled remote plasma source, may also be coupled betweenthe gas source and the backing plate. In one process configuration,between processing substrates, a cleaning gas may be provided to theremote plasma source 424 so that a remote plasma is generated andprovided to clean chamber components. The cleaning gas may be furtherexcited by the RF power source 422 provided to the showerhead. Suitablecleaning gases include but are not limited to NF₃, F₂, and SF₆. Examplesof remote plasma sources are disclosed in U.S. Pat. No. 5,788,778 issuedAug. 4, 1998 to Shang et al, which is incorporated by reference to theextent not inconsistent with the present disclosure.

In one embodiment, the heating and/or cooling elements 439 may be set toprovide a substrate support temperature during deposition of about 400°C. or less, preferably between about 100° C. and about 400° C., morepreferably between about 150° C. and about 300° C., such as about 200°C. The spacing during deposition between the top surface of a substratedisposed on a substrate carrier 425 disposed on the substrate receivingsurface 432 and the showerhead 410 may be between 400 mil and about1,200 mil, preferably between 400 mil and about 800 mil.

FIG. 5 is a top schematic view of one embodiment of a processing system,or system 500, having a plurality of process chambers 531-537, such asPECVD chambers chamber 400 of FIG. 4 or other suitable chambers capableof performing the processes described in conjunction with FIG. 3. Thesystem 500 includes a transfer chamber 520 coupled to a load lockchamber 510 and the process chambers 531-537. The load lock chamber 510allows substrates to be transferred between the ambient environmentoutside the system and vacuum environment within the transfer chamber520 and process chambers 531-537. The load lock chamber 510 includes oneor more evacuatable regions that is configured to hold one or moresubstrate carriers 425 that are configured to support a plurality ofsubstrates 210. The evacuatable regions are pumped down during the inputof the substrates into the system 500 and are vented during the outputof the substrates from the system 500. The transfer chamber 520 has atleast one vacuum robot 522 disposed therein that is adapted to transferthe substrate carriers 425 and substrates between the load lock chamber510 and the process chambers 531-537. Seven process chambers are shownin FIG. 5; however, the system 500 may have any suitable number ofprocess chambers.

In one embodiment of system 500, a first processing chamber 531 isconfigured to perform the first clean process performed at box 303, asecond process chamber 532 is configured to perform the processperformed at box 304, a third process chamber 533 is configured toperform the process performed at box 305, and a fourth process chamberis configured to perform the process performed at box 308 on thesubstrates. In another embodiment of system 500, a first processingchamber 531 is configured to perform the first clean process performedat box 303, a second process chamber 532 is configured to perform theprocess performed at box 306, and a third process chamber 533 isconfigured to perform the process performed at box 308 on thesubstrates. In yet another embodiment of system 500, a first processingchamber 531 is configured to perform the process performed at box 302and a second process chamber 532 is configured to perform process theprocess performed at box 308 on the substrates. In yet anotherembodiment of system 500, at least one of the process chambers 531-537is configured to perform process the process performed at box 302 andprocess the process performed at box 308 on the substrates.

Clean Processes

Referring back to FIG. 3, during the first part of the process sequence300 the surfaces of the substrate 210 are subjected to a plurality ofprocessing steps that are used to remove the oxide layer 215, remove thedead region 216, and form a charged layer 219 on the surfaces of thesubstrate. The following are illustrative examples of one or more of theprocesses performed at boxes 302-308 that may be performed in aprocessing chamber, similar to process chamber 400 discussed above. Theprocesses described below generally include methods of preparing asurface of a substrate using primarily dry processing techniquesperformed in one or more process chambers (e.g., process chamber 400)found in one or more cluster tools, such as systems 500.

As noted above, during prior processing steps or exposure to oxygen asubstrate 210 may acquire an oxide layer 215 and a dead region 216 maybe formed. In many embodiments, this will occur after formation of thelast layer of a solar cell junction, such as a p-type or n-type dopedlayer. In other embodiments, this will occur prior to forming one ormore conductor layers, such as after a heavily doped, or degenerativelydoped, p-type layer is formed. It should be noted that while the variousembodiment of the invention described herein are discussed in relationto cleaning a surface of a deposited layer, such as the emitter region202, this configuration is not intended to limit the scope of theinvention, since the apparatus and cleaning process(es) described hereincan be used during any phase of the solar formation process withoutdeviating from the basic scope of the invention described herein.

In one embodiment, at box 303, after disposing one or more of thesubstrates 210, which are on a substrate carrier 425 positioned on thesupport 430 in a process chamber 400, the oxide layer 215 is exposed toa reactive gas to form a thin film (not shown) on the oxide layer 215.The reactive gas may comprise nitrogen, fluorine, and hydrogen. In someembodiments, the reactive gas comprises radicals containing nitrogen,fluorine, or both, and is provided to the process chamber having thesubstrate disposed therein and directed toward the substrate. The thinfilm generally comprises a solid compound formed by reaction of theradicals with oxygen from the oxide layer 215.

Next, the thin film formed on the oxide layer 215 is thermally treatedto remove it from the surface of the substrate. In some embodiments, thethermal treatment may be an annealing process performed in theprocessing chamber 400, or another adjacent chamber found in the system500. During this step the thin film sublimes away from the substratesurface, taking oxygen and other impurities, and leaving ahydrogen-terminated layer (not shown) behind. In some embodiments, thehydrogen-terminated layer may also have traces of fluorine atoms in theformed hydrogen-terminated layer. Preferably, a temperature of about 75°C. or higher is used to cause the sublimation of the created thin film.

An exemplary reactive cleaning process for removing native oxides on asurface of the substrate using an ammonia (NH₃) and nitrogen trifluoride(NF₃) gas mixture performed within a processing chamber will now bedescribed. The reactive cleaning process begins by placing a substrateinto a processing chamber. During processing, the substrate may becooled below about 65° C., such as between about 15° C. and about 50° C.Typically, the substrate support is maintained below about 22° C. toreach the desired substrate temperatures. In some embodiments, it isuseful to maintain a temperature of the substrate below a temperature ofthe chamber walls during formation of the thin film to preventcondensation of reactive species from the reactive gas on the chamberwalls.

A precursor gas mixture comprising ammonia gas and nitrogen trifluoridegas is introduced into the process chamber to form a cleaning gasmixture. The amount of each gas introduced into the chamber is variableand may be adjusted to accommodate, for example, the thickness of theoxide layer to be removed from the substrates, the geometry of thesubstrates being cleaned, the volume capacity of the reactive gasmixture and the volume capacity of the chamber body. In one aspect, thegases are added to provide a precursor gas mixture having at least a 1:1molar ratio of ammonia to nitrogen trifluoride. In another aspect, themolar ratio of the gas mixture is at least about 3 to about 1 (ammoniato nitrogen trifluoride). Preferably, the gases are introduced in thedry etching chamber at a molar ratio of between about 1:1 (ammonia tonitrogen trifluoride) and about 30:1, more preferably, between about 5:1(ammonia to nitrogen trifluoride) and about 10:1.

A purge gas or carrier gas may also be added to the precursor gasmixture. Any suitable purge/carrier gas may be used, such as argon,helium, hydrogen, nitrogen, forming gas, or mixtures thereof. Typically,the volume fraction of ammonia and nitrogen fluoride in the precursorgas mixture ranges from about 0.05% to about 20%. The remainder of theprecursor gas mixture may be the carrier gas. In one embodiment, thepurge or carrier gas is first introduced into the chamber body beforethe reactive gases to stabilize the pressure within the chamber body.

The operating pressure within the chamber body can vary. The pressuremay be maintained within a range from about 500 mTorr to about 30 Torr,preferably from about 1 Torr to about 10 Torr, and more preferably fromabout 3 Torr to about 6 Torr, such as about 3 Torr. Dissociative energyis applied to the precursor gas mixture to form a reactive gas mixture.A RF power within a range from about 0.01 W/cm² to about 0.74 W/cm² maybe applied to ignite a plasma of the precursor gas mixture within theplasma cavity (e.g., processing region 406 in FIG. 4). In one example,the frequency at which the RF power is applied is very low, such as lessthan about 100 kHz, and more preferably, within a range from about 50kHz to about 90 kHz. In most embodiments, the surface of the substratewill be etched (i.e., converted into a thin film of anneal precursor) ata rate of between about 3 Å/sec and about 10 Å/sec, such as about 5Å/sec.

The plasma energy dissociates the ammonia and nitrogen trifluoride gasesinto reactive species that combine to form a highly reactive ammoniafluoride (NH₄F) compound and/or ammonium hydrogen fluoride (NH₄F—HF)which reacts with the substrate surface. The gases dissociate to formcharged and uncharged reactive species. In one embodiment, the carriergas is first introduced into the dry etch chamber, a plasma of thecarrier gas is generated, and then the reactive gases, ammonia andnitrogen trifluoride, are added to the plasma.

Not wishing to be bound by theory, it is believed that the etchant gas,NH₄F and/or NH₄F—HF, reacts with the native oxide surface to formammonium hexafluorosilicate ((NH₄)₂SiF₆), ammonia (NH₃), and water(H₂O), while releasing oxygen gas (O₂). The ammonia and water are vaporsat processing conditions and removed from the chamber by a vacuum pumpattached to the chamber. A thin film of ammonium hexafluorosilicate isleft behind on the substrate surface. The reaction mechanism can besummarized as follows:

NF₃+2NH₃→NH₄F+2HF+N₂

6NH₄F+SiO₂→(NH₄)₂SiF₆+H₂O

The thin film of ammonium hexafluorosilicate on the substrate surfacemay be removed to expose the underlying substrate surface during ananneal process. In one embodiment, the processing chamber radiates heator provides an inert gas containing RF plasma to dissociate or sublimatethe thin film of ammonium hexafluorosilicate into volatile SiF₄, NH₃,and HF products (e.g., (NH₄)₂SiF₆+heat.→NH₃+HF+SiF₄). These volatileproducts are then removed from the chamber by the vacuum pump attachedto the system. In one example, a substrate temperature of about 75° C.or higher is used to effectively sublime and remove the thin film fromthe substrate. Preferably, a temperature of about 100° C. or higher isused, such a temperature within a range from about 115° C. to about 300°C., such as about 120° C. Higher temperature promotes fastersublimation. In one embodiment, the gas distribution plate is heated toa temperature of about 180° C. and spaced about 100 mils from thesubstrate to anneal the substrate. Once the film has been removed fromthe substrate, the chamber is purged and evacuated prior to removing thecleaned substrate. During the anneal process, the substrate may bemaintained under vacuum, or may be exposed to hydrogen gas, or hydrogengas or inert gas plasma, depending on the embodiment. Removal of thethin film removes oxygen from the substrate surface and depositshydrogen, fluorine, or both on the substrate surface. In someembodiments, the anneal process may be performed on a side of thesubstrate opposite the side on which the cleaning film is deposited byuse of heating elements found in the susceptor support. For example, ifa first side of the substrate is subjected to the radical containing dryclean process described above, wherein a thin film is formed on thefirst side, a second side of the substrate, opposite the first side, maybe heated to perform the anneal process.

In some embodiments of the process performed at box 304, it may beuseful to supplement the process gas delivered during the processperformed at box 303 with hydrogen (H₂) to subsequently remove the deadregion 216. The addition of hydrogen to the process gas delivered duringthe process performed at box 303 promotes the concentration of hydrogenradicals and hydrogen fluoride in the reactive gas. The presence ofthese two species in the reaction mix will etch portions of dopedsilicon layers, such as the dead region 216, while the ammonium fluoridespecies form hexafluorosilicate on the surface of the substrate. Thismay improve cleaning, and may also effect removal of dopants from asurface of the substrate. Therefore, in some embodiments of the processperformed at box 304, or the dead region 216 removal step, a molar ratioof H₂:NH₃ between about 0.1 and about 1.0 may be used to promote modestetching of the surface, with higher ratio causing more or fasteretching. The molar ratio of hydrogen to ammonia in the reactive gasmixture controls the selectivity of the gas for etching doped siliconlayers versus silicon oxide layers. Increasing the molar ratio ofhydrogen to ammonia results in faster etching of the doped layerrelative to the oxide layer, and vice versa. Addition of hydrogen mayalso allow selective etching of undoped silicon layers relative to oxidebecause the hydrogen forms HF in the reactive gas mixture, which etchesundoped silicon layers.

Nitrogen and hydrogen gas may be substituted for ammonia in someembodiments. When dissociated, nitrogen and hydrogen may combine to formthe ammonium radicals discussed above. Providing hydrogen and nitrogenin a molar ratio of about 3:1 will approximate the effect of ammonia atcertain pressures and power levels. Varying the ratio may havebeneficial side-effects, depending on the embodiment. For example, whencleaning a doped or heavily doped surface, a higher proportion ofhydrogen may improve the etch rate of the doped surface by providingmore hydrogen radicals to remove dopants.

Next, in the process performed at box 305, the substrate is exposed to areactive gas containing RF plasma that is used to form a negativelycharged layer 219 on the exposed surfaces of the substrates. In oneexample, the substrate is exposed to a 13.56 MHz RF plasma that containsan amount of a halogen gas. In one embodiment, the halogen gas is afluorine gas or chlorine gas, which is provided from a gas comprisingnitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆), fluorine gas(F₂), chlorine gas (Cl₂), hydrochloric acid (HCl),dichlorodifluoromethane (CCl₂F₂), or fluorocarbons, such as C₂F₂, CF₄,C₂F₄, and C₃F₆. In one example, the halogen gas is diatomic fluorine gas(F₂). It is believed that by exposing the cleaned surface, such assurface 205, directly to a plasma containing ionized fluorine, and/orionized chlorine, the exposed surface can be “doped”, “stuffed” orcovered with a fluorine rich layer, or chlorine rich layer, that has anegative charge. In some configurations, it is desirable to activelybias the substrate to “dope”, “stuff” or cover the surface with thefluorine rich, or chlorine rich, layer that has a negative charge. Inone example, by applying an RF bias to a showerhead 410 and groundingthe substrate support 430 the ionized species found in the plasma can bedriven to the surface of the substrates to “dope”, “stuff” or cover thesurface with the fluorine rich, or chlorine rich, layer that has anegative charge. In one example, the process performed at box 305comprises delivering 1500 sccm/L of a fluorine (F₂) gas and 500 sccm/Lof argon to achieve a chamber pressure of about 2000 mTorr, supplyingabout 0.6 mW/cm² of 13.56 MHz of RF power to showerhead for about 15seconds to form a 3 to 10 angstrom (Å) thick charged layer 219 that hasa charge density greater than about 1×10¹² charge/cm². In oneembodiment, it is desirable to purge the process chamber and not flow ahydrogen containing gas into the processing region 406 during theprocess performed at box 305 to avoid the formation of certain types ofetchants, such as hydrofluoric acid (HF), which can affect thedeposition and properties of the formed charged layer 219.

Next, in the process performed at box 308, the substrate is exposed to areactive gas containing RF plasma that is used to form a passivationlayer, such as a multilayer hydrogenated SiN film on the substrates 210.FIG. 6 illustrates an exemplary process sequence 600 used to form thepassivation layer deposited in box 308 on a solar cell substrate 210.

In one embodiment, at box 602, after the substrates 210 are positionedin another of the processing chambers 531-537 in the processing system500, or, alternately, the same processing chamber used to form one ormore of the prior steps, a process gas mixture is flowed into thechamber. The process gas mixture includes a precursor gas mixture and ahydrogen gas (H₂) diluent. The hydrogen gas diluent may have a flow rateas high as approximately two times the flow rate of the precursor gasmixture. The precursor gas mixture may be a combination of silane (SiH₄)and nitrogen (N₂), silane and ammonia (NH₃), or silane, ammonia, andnitrogen. In one example, flow rates for a first process gas mixturecontaining silane, ammonia, and hydrogen may be 3.5 sccm, 50 sccm, and80 sccm, per liter of chamber volume, respectively. Alternately, flowrates for a first process gas mixture containing silane, ammonia,nitrogen, and hydrogen may be 5 sccm, 16 sccm, 40 sccm, and 80 sccm, perliter of chamber volume, respectively. The substrate support 430temperature is generally maintained at a temperature of about 390° C.during this process step.

Next, at box 604, a plasma is then generated in the processing chamberto deposit a SiN layer on the substrates 210, wherein the SiN layer issuitable for use as a combined ARC and passivation layer for a solarcell. Namely, the SiN layer so deposited has a mass density of betweenabout 2.0 and 2.8 g/cm³ (e.g., mass density between about 2.6 and 2.8g/cm³), a refractive index of between about 2.0 and 2.6, and a hydrogenconcentration of between about 5 atomic percent and 15 atomic percent.In one embodiment, a chamber pressure of 1.5 Torr may be maintained inthe chamber and an RF power intensity of 0.74 W/cm² at a frequency of13.56 MHz is applied to the showerhead 410 of the processing chamber 400to generate a plasma for a period of time of about 9 seconds, while thefirst process gas mixture is delivered to the processing region 406. Inone example, the processing chamber in which the ARC layer is formed isa plasma enhanced chemical vapor deposition (PECVD) chamber.

In one configuration, as discussed above, the layer formed in box 604 ispart of a plurality of passivation layers, such as passivation layers221 and 222, which are used to passivate the surface of the substrate.In one example, a passivation layer 221 is formed in box 604, which is aSiN layer that has a mass density of between about 2.6 and 2.8 g/cm³, arefractive index of between about 2.4 and 2.6, and a hydrogenconcentration of between about 5 atomic percent and 15 atomic percent.

Next, at box 606, a flow of the first process gas mixture is stopped,and a second process gas mixture is delivered into the chamber. In oneexample, the second process gas mixture may contain 5.5 sccm of silane(SiH₄), 16 sccm of ammonia (NH₃), and 40 sccm of nitrogen (N₂), perliter of chamber volume. In one embodiment, the plasma created inprocess(es) performed at box 604 is extinguished in the processingchamber and the flow of the first process gas mixture is stopped, beforethe second process gas mixture is introduced into the processingchamber. In one embodiment, the process “break” performed at box 606lasts about 2 seconds. In this case, the first process gas mixture maybe substantially purged from the chamber before the second process gasmixture is flowed into the chamber. The substrate support 430temperature is generally maintained at a temperature of about 390° C. Inone example, a PECVD deposition process is performed

Finally, at box 608, a bulk SiN layer, or passivation layer 222, isdeposited over the interface layer to form a dual stack SiNARC/passivation layer on the substrates 210. In this way, the majorityof the SiN passivation layer may be deposited by a substantially fasterprocess without affecting the quality of solar cell passivation. If theplasma is extinguished in the chamber prior to the introduction of thesecond process gas mixture, then plasma is re-ignited to enabledeposition of the bulk SiN layer. In one embodiment of the process 608,a chamber pressure of 1.5 Torr may be maintained in the processingchamber and an RF power intensity of 0.74 W/cm² at a frequency of 13.56MHz is applied to the showerhead 410 of the processing chamber 400 togenerate a plasma for a period of time of about 15 seconds, while thesecond process gas mixture is delivered to the processing region 406. Inone example, the deposited bulk SiN layer has a mass density of betweenabout 2.3 and 2.6 g/cm³, a refractive index of between about 2.05 and2.15, and a hydrogen concentration of between about 10 atomic percentand 25 atomic percent. In one example, the refractive index (n) value ofthe passivation layer 221 is 2.4 and that of the passivation layer 222is 2.08, while the surface of the substrate 210 typically has arefractive index of about 3.0.

After performing the process sequence 300 processing steps the substratemay then be further processed to form a solar cell device that can beconnected to an external grid that is adapted to collected the generatedelectrical current. In one example, additional metal and/or dielectriclayers may be formed on the either side of the processed substrate(e.g., front surface 205, rear surface 206) to form the various solarcell device interconnect structures.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of passivating a surface a solar cell substrate, comprising:exposing a surface of a p-type doped region formed on a substrate to afirst RF plasma that comprises a first processing gas and a firstfluorine containing gas; exposing the surface of the p-type doped regionto a second RF plasma that comprises a halogen gas; and RF biasing thesubstrate, during at least a portion of the exposing the surface to thesecond RF plasma, to form a negatively charged layer on the surface. 2.The method of claim 1, wherein the first processing gas comprisesammonia.
 3. The method of claim 1, further comprising exposing thesurface of the p-type doped region to a third RF plasma that comprises asecond processing gas, a second fluorine containing gas and hydrogencontaining gas before exposing the surface of the p-type doped region tothe second RF plasma.
 4. The method of claim 3, wherein the first andthe second processing gases comprise ammonia, and the first and thesecond fluorine containing gases further comprise nitrogen trifluoride,and wherein a molar ratio of ammonia to nitrogen trifluoride in thefirst and the second processing gases is between about 1:1 and 30:1. 5.The method of claim 1, wherein the halogen gas comprises fluorine orchlorine.
 6. The method of claim 1, further comprising depositing afirst silicon nitride-containing layer on the surface of the p-typedoped region after forming the negatively charged layer on the surface.7. The method of claim 6, further comprising depositing a second siliconnitride-containing layer on the first silicon nitride-containing layer.8. The method of claim 1, wherein the second RF plasma does not containa hydrogen containing gas.
 9. The method of claim 1, wherein theexposing the surface to the first RF plasma is performed in a firstprocessing chamber; and the method further comprises: heating thesubstrate to a temperature greater than about 75° C. in a secondprocessing chamber before exposing the surface of the p-type dopedregion to the second RF plasma; and transferring the substrate betweenthe first and the second processing chambers in a controlledenvironment.
 10. A method of passivating a surface a solar cellsubstrate, comprising: exposing a surface of a p-type doped regionformed on an n-type substrate to a first RF plasma that comprises afirst processing gas and a first fluorine containing gas; exposing thesurface of the p-type doped region to a second RF plasma that comprisesa second processing gas, a second fluorine containing gas and hydrogencontaining gas; exposing the surface of the p-type doped region to athird RF plasma that comprises a halogen gas; RF biasing the substrate,during at least a portion of the exposing the surface to the third RFplasma, to form a negatively charged layer on the surface of the p-typedoped region; and depositing a first silicon nitride-containing layer onthe formed negatively charged layer.
 11. The method of claim 10, whereinthe first and the second processing gases comprise ammonia, the firstand the second fluorine containing gases comprise nitrogen trifluoride,and wherein a molar ratio of ammonia to nitrogen trifluoride in thefirst and the second processing gases is between about 1:1 and 30:1. 12.The method of claim 11, wherein the halogen gas comprises fluorine orchlorine.
 13. The method of claim 11, further comprising depositing asecond silicon nitride-containing layer on the first siliconnitride-containing layer.
 14. A method of passivating a surface a solarcell substrate, comprising: removing an oxide layer from a surface of ap-type doped region formed on a substrate; removing a portion of thesurface of the substrate that has a concentration of p-type atoms thatis greater than the average concentration of the p-type atoms in thep-type doped region; and forming a negatively charged layer on thesurface by exposing the surface to an RF plasma comprising fluorine orchlorine.
 15. The method of claim 14, wherein forming the negativelycharged layer is performed after removing the portion of the surface ofthe substrate, and before exposing the substrate surface to an oxygencontaining gas.
 16. The method of claim 14, wherein removing the aportion of the surface and forming the negatively charged layer furthercomprises: positioning two or more of the substrates on a substratecarrier; positioning the two or more substrates and the substratecarrier in a processing region of a plasma processing chamber; andremoving a portion of the surface of the substrate and forming thenegatively charged layer on the two or more substrates simultaneouslyusing a capacitively coupled plasma generated over the processingsurfaces.
 17. The method of claim 14, wherein removing the oxide layerfrom the surface comprises: immersing the substrate in a solutioncomprising hydrofluoric acid; and rinsing the substrate prior to formingthe negatively charged layer.
 18. The method of claim 14, wherein themethod is performed in the same processing chamber.
 19. The method ofclaim 14, further comprising: depositing a first siliconnitride-containing layer on the negatively charged layer; and depositinga second silicon nitride-containing layer on the first siliconnitride-containing layer.
 20. The method of claim 19, wherein the p-typedopant in the p-type doped region comprises boron, and the removing aportion of the surface of the substrate is performed until the surfacecomprises at least 85% active boron.